We initiated a new field of yeast genetics with our discovery that two non-chromosomal genetic elements, [URE3] and [PSI], were prions (infectious proteins), analogous to the agent believed to cause the transmissible spongiform encephalopathies of mammals. The first, [URE3], is an altered form of Ure2p, the protein product of the chromosomal URE2 gene important in regulation of nitrogen catabolism. The second, [PSI], is an altered form of Sup35p, a subunit of the translation release factor and product of the chromosomal SUP35 gene. We found that Ure2p is more resistant to protease digestion in [URE3] strains than in wild-type strains, and is aggregated specifically in cells carrying the prion, supporting the prion model and suggesting amyloid formation as its molecular basis. The N-terminal 65 aminoacid residues of Ure2p is sufficient to propagate [URE3], or to induce the de novo appearance of [URE3]. Certain deletions within the C-terminal nitrogen regulation domain of Ure2p result in a 100-fold increase the frequency with which the [URE3] prion arises, indicating the the prion domain is stabilized in its normal form by interaction with the C-terminus. We showed that the Ure2p prion domain forms amyloid filaments in vitro. Moreover, just as the prion domain induces prion formation in vivo, it induces the otherwise stably soluble native Ure2p to form amyloid in vitro. The properties of Ure2p amyloid formation in vitro reflect and explain the prion properties of [URE3] in vivo. We thus proposed that the [URE3] prion is an infectious amyloidosis. We showed that fragments of Ure2p or fusions with other proteins cure the prion efficiently. This phenomenon may be due to interruption of the growth of the amyloid 'crystals' due to the fragments or fusion proteins, and suggests a new approach to the treatment of amyloid diseases. We find that the Mks1 protein is essential for the de novo formation of the [URE3] prion. Mks1 activity is negatively regulated by the Ras - cAMP pathway, and we find that activation of Ras2p prevents de novo [URE3] prion formation by inactivating Mks1. We showed that the Hsp104 chaperone is necessary for [URE3] prion propagation, and that overexpression of the Hsp40-family chaperone Ydj1p can cure the [URE3] prion. In collaboration with Drs. Vladislav Speransky and Alasdair Steven (NIAMS), we further showed that cells with the [URE3] prion contain networks of filaments consisting of the Ure2 protein. Such filamentous networks were not present in cells without the prion. Further, most of the Ure2p in extracts of [URE3] strains is in a form insoluble even after boiling in 3M urea and 2% SDS, confirming that it is in an amyloid state. Our collaborators, Drs. Tim Umland and David Davies (LMB, NIDDK), have determined the structure of the nitrogen regulation domain of Ure2p and find that it is closely similar to glutathione-S-transferases (GST). Ure2p is inactivated by prion (amyloid) formation in vivo. We find that fusion proteins of the Ure2 prion domain with each of several enzymes, including GST (highly homologous to Ure2p) form amyloid filaments. In these amyloid filaments, the enzyme part is fully active. This suggests that Ure2p is not inactivated by a conformational change in the functional part of the molecule, but by a steric effect or diffusion limitation on the interaction of Ure2p with Gln3p. The Ure2-GFP fusion protein forms amyloid filaments with a helical form. The length of the helical repeat is constant within each filament, but this length varies by more than 2 fold from one filament to another. This may be the basis of prion strains, that have different infectious properties and different effects on the host. We have isolated homologs of the URE2 gene from other strains of S. cerevisiae, from various pathogenic Candida species and from a filamentous fungus. While the C-terminal domain is highly conserved and the homologs can substitute for the cerevisiae Ure2p, the N-terminal domain (up to residue 99) is highly variable. Nonetheless, there is a conserved part of the prion domain from residues 10 to 39. This region apparently interacts with the Ure2p C-terminus as judged by inactivation of Ure2p when the fragment is overexpressed. This region also is responsible for the curing of the [URE3] prion by fusions with GFP mentioned above. Current efforts are underway to determine the role of this region in prion formation and propagation and in the function of Ure2p. Other efforts are underway to identify new prions using functional screens.